libct/userns: make fuzzer Linux-only, and remove stub for uidMapInUserNS

The fuzzer for this only runs on Linux; rename the file to be Linux-only
so that we don't have to stub out the uidMapInUserNS function.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

libcontainer/userns/userns_linux_fuzzer.go

@@ -1,4 +1,4 @@
-//go:build gofuzz
+//go:build linux && gofuzz
 
 package userns
 

libcontainer/userns/userns_unsupported.go

@@ -7,9 +7,3 @@ package userns
 func runningInUserNS() bool {
 	return false
 }
-
-// uidMapInUserNS is a stub for non-Linux systems
-// Always returns false
-func uidMapInUserNS(uidMap string) bool {
-	return false
-}